Question

可能重复：
Why does C# execute Math.Sqrt() more slowly than VB.NET?

我遇到了一个有趣的问题，其中我在VB.net中有代码，在C ++中有完全相同的代码。我希望C ++自然比VB.net运行得更快，但我得到的恰恰相反：VB.net的运行速度是C ++的两倍多。程序遍历从1到2,000,000的所有数字，确定它们是否为素数，并将所有素数加在一起。以下是以下片段：

C ++

void problem10(void)
{   
   clock_t init, final;
   init=clock();

   int maxVal = 2000000;
   long long sumOfPrimes = 0;
   for (long i = 2; i < maxVal; i++)
   {
      if (isPrime(i))
      {
         sumOfPrimes+= i;
      }
   }
   final = clock() - init;
   cout << (double)final / ((double)CLOCKS_PER_SEC);
   cout << "The sum of all the prime numbers below " << maxVal << " is " << sumOfPrimes;
}

bool isPrime(int NumToCheck)
{
   for (int i = 2; i <= (sqrt((double)NumToCheck)); i++)
   {
      if (NumToCheck % i == 0)
      {
         return false;
      }
   }
   return true;
}

C ++输出：

3.846The sum of all the prime numbers below 2000000 is 142913828922

这是完全相同的事情，只用VB.net编写

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    Dim watch As New Stopwatch
    watch.Start()

    Dim maxVal As Long = 2000000
    Dim sumOfPrimes As Long = 0
    For i As Integer = 2 To maxVal
        If (isPrime(i) = True) Then
            sumOfPrimes += i
        End If
    Next
    watch.Stop()
    Console.WriteLine(watch.ElapsedMilliseconds)
    Console.WriteLine("The sum of all the prime numbers below " & maxVal & " is " & sumOfPrimes)
End Sub

Function isPrime(ByVal NumToCheck As Integer) As Boolean
    For i As Integer = 2 To (Math.Sqrt(CDbl(NumToCheck)))
        If (NumToCheck Mod i = 0) Then
            Return False
        End If
    Next
    Return True
End Function

VB输出：

1643
The sum of all the prime numbers below 2000000 is 142913828922

我觉得有一些明显的东西让我失踪，因为我真的看不到VB.net比C ++更快。有什么想法吗？

Answer 1

VB.Net解决方案在循环开始时计算一次平方根，而C ++（以及C和C＃和Java等）都通过循环计算平方根，因为它们的循环基元定义不同

Answer 2

你的问题是那个

For i As Integer = 2 To (Math.Sqrt(CDbl(NumToCheck)))

与

不同

for (int i = 2; i <= (sqrt((double)NumToCheck)); i++)

VB.Net将在开始时对Math.Sqrt进行一次评估，而c ++必须在每次迭代时对其进行评估。在C＃而不是c ++的stackoverflow上已经存在同样的问题。

VB.net比C ++更快？

2 个答案: